Hot rolled structural steel oxide scale structure and its influence on pickling quality

Summary

In order to study the thickness and structure of iron oxide scale in hot-rolled structural steel and its impact on pickling quality, two hot rolling processes, "high temperature, low cooling rate" and "low temperature, high cooling rate", were designed, and the production and preparation of hot-rolled structural steel were carried out. The thickness distribution, structural characteristics, and surface morphology of oxide scale in the width direction of the strip steel were analyzed using optical microscopy and scanning electron microscopy; Based on the differences in thickness and structure of oxide scale under different hot rolling processes, the pickling effect of strip steel and the mechanism of pickling color difference defects were discussed. The results indicate that the oxide scale of hot-rolled structural steel is composed of low-temperature transformation structures of outer Fe3O4 and inner FeO. Due to the uneven distribution of temperature field during hot rolling and coiling, the thickness of oxide scale at the edge of the strip is small, the structure is dense, and the content of Fe3O4 is higher than that in the middle; The thickness of the oxide scale in the middle of the strip steel is large, the structure is loose, and the low-temperature transformation of the FeO structure is more complete than at the edges. Under the hot rolling mode of "high temperature and low cooling rate", the transverse uniformity of the oxide scale is poor, and the thickness difference between the edges and the middle is 4.7 μ m. The thickness difference of Fe3O4 layer is 2.5 μ M. Under the "low temperature, high cooling rate" hot rolling mode, the horizontal uniformity of the oxide scale is good, and the thickness of the oxide scale in different parts differs by 2.5 μ m. Compared to the former, it has decreased by 46.81%, while the thickness of Fe3O4 layer differs by 0.2 μ m. Only 8.0% of the former. The horizontal differences in thickness and structure of iron oxide scale are the main causes of color difference defects in strip steel pickling. Eutectoid organization（ α- Fe+Fe3O4 is loose and porous, and its pickling process is carried out in a "stripping" manner, resulting in higher pickling efficiency. The area where the strip steel is preferentially pickled continues to come into contact with the acid solution, generating a large number of "erosion cracks" on the surface, forming macroscopic color differences. Improving the distribution of iron oxide scale and controlling the proportion of Fe3O4 and eutectoid structure in iron oxide scale are the key to optimizing production processes and improving pickling quality.

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Keywords

Hot rolled strip steel; Hot rolling process; Iron oxide scale; Acid washing process; surface quality 

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Introduction

QStE series strip steel belongs to hot-rolled structural steel for cold forming. This type of product has high strength and good formability, and is mostly used in the manufacturing of key components such as automotive structural components and chassis components. In actual production, surface defects during pickling are the main reason for the degradation of such products, with the most representative being the problem of color difference during pickling. This defect will deteriorate the coating performance of the strip steel and cause production losses in the coating process.

In recent years, people have done a lot of work in the composition design, process optimization, and performance improvement of hot-rolled structural steel, but there is still insufficient research on acid pickling quality based on iron oxide scale control. Wu Hanke et al. discussed the surface indentation problem during the processing and forming process based on the structural characteristics of iron oxide scales under different hot rolling conditions. Dong Hanjun et al. established a pickling water bath model and studied the pickling effect of different iron oxide scales by adjusting the acid concentration and pickling time. However, this method is difficult to characterize the physical quality of the product. Cao Guangming et al. studied the pickling mechanism of iron oxide scale mainly composed of FeO/Fe3O4, without exploring the quality control of pickling.

Under certain conditions of pickling process, the quality of pickling is directly related to the state of surface oxide scale. Studying the control process of iron oxide scale and clarifying the relationship between iron oxide scale and pickling quality is crucial for reducing pickling defects in strip steel.

This article starts with the problem of color difference during pickling of hot-rolled structural steel QStE420TM, analyzes the structure of oxide scale in the width direction of the strip and the surface morphology of pickling, and clarifies the mechanism of color difference during pickling. Explored the improvement plan for pickling quality from the perspective of iron oxide scale structure, thereby achieving effective control of pickling defects.

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Conclusion

(1) The iron oxide scale on the strip steel is composed of outer Fe3O4, inner eutectoid structure, and a small amount of residual FeO. From the edge to the middle, the Fe3O4 content gradually decreases. Under high temperature and slow cooling conditions, the volume fraction of Fe3O4 in the middle oxide scale is less than 5%, while the eutectoid transformation of the FeO layer is more complete. The thickness of the oxidized iron sheet tends to stabilize after being no less than 70 mm away from the edge.

(2) Under the "high temperature, low cooling rate" mode, the transverse uniformity of the oxide scale on the strip steel is poor, with a thickness difference of 4.7 between the edge and middle oxide scales μ m. The thickness difference of Fe3O4 layer is 2.5 μ M. In the "low temperature, high cooling rate" mode, the lateral uniformity of the oxide scale is significantly improved, and the thickness difference of the oxide scale is reduced by 46.81% compared to the former, while the thickness difference of the Fe3O4 layer is only 8.0% of the former.

(3) The defect in black steel is caused by the uneven distribution of iron oxide scale, which leads to color difference during acid washing. The formation of this defect is directly related to the thickness and structure of the iron oxide scale, and is present in the loose eutectoid transformation structure α- The presence of Fe and microcracks is beneficial for improving the pickling efficiency, while the dense Fe3O4 layer will hinder the pickling process. Improving the horizontal distribution uniformity of iron oxide scale, controlling the structure and corresponding tissue proportion of iron oxide scale, are the key to optimizing production processes and improving pickling quality.

(4) The temperature distribution during the production process affects the formation of iron oxide scales. The transverse temperature difference before precision rolling of the strip exceeds 100 ℃, and the longitudinal temperature difference of the incoming steel coil reaches 92 ℃. The precision rolling temperature determines the Fe3O4 content formed by high-temperature oxidation, and the temperature difference after coiling leads to different contents of the low-temperature transformation structure of transverse FeO.